June 2020, Vol. 247, No. 6
Features
Operating, Installation Considerations For Pipeline Composite Repairs
By Colton Sheets, Senior Associate, Stress Engineering Services, Inc.
Composite repairs are no longer new to the pipeline integrity world. A significant body of work has been developed over the last several decades characterizing their performance and developing guidelines for end-users.
Empirical and analytical studies have been funded by government agencies, composite manufactures, end-users and collaborative industry programs. Because of this effort, applications for composite repairs continue to advance as pipeline operators are provided with greater flexibility in remediating defects.
Over this time period, composite repairs have been installed across the globe, in almost every imaginable scenario, providing operators with an alternative solution to clamps, steel sleeves or pipeline replacement.
Through the development and installation of these systems, composite repair manufacturers have provided a critical service that keeps pipelines operating safely. Although success has been the primary outcome when making these repairs, like all industries, there have been occasional setbacks and shortcomings.
These serve as reminders that not all repair scenarios are created equally and that fundamental first principles must always be considered. A perfectly designed composite repair with the most advanced materials can still be rendered completely ineffective if not installed correctly.
Unlike steel sleeves, the nonmetallic materials used in most pipeline composite repair applications cannot easily be inspected and qualified according to industry standards. Therefore, special consideration must be given before, during and after the installation to ensure the chosen composite repair has the best chance to perform as intended for the requisite life.
Since it is always important to be reminded of the first principles that guide success and failure, this article will highlight some easily overlooked design considerations at three key points in the life of a composite repair:
- Surface preparation prior to installation
- Operating conditions during installation
- Operating conditions following installation
In many cases, the industry-recognized standards for pipeline composite repairs (ASME PCC-2 and ISO 24817) provide subtle, and not so subtle, guidance for each of these stages in a composite repair’s life. In other instances, the ever-popular “engineering judgment” must be relied upon to ensure the success of the composite repair solution.
It first must be understood that the majority of successful composite repairs are nominally able to do two things simultaneously:
- A successful repair system serves as a protective barrier between the substrate (pipe being repaired) and the external environment that, in some cases, is the root cause of the targeted defect.
- A successful composite repair provides sufficient reinforcement to the defect area while not exceeding its own strength limitations.
The reinforcement described in Item 2 is what allows an affected pipeline, in many cases, to continue operating at internal pressures appropriate for nominal, undamaged pipe. It does this by keeping the targeted defect below critical stress and strain thresholds for remaining strength and remaining life.
Spending sufficient time and energy understanding these requirements in the design and installation phases will significantly increase the likelihood of success for a given repair. A general understanding of typical composite systems is necessary to connect the installation and operation variables and their influence on performance.
Overview
A composite, by definition, is a material that consists of multiple, distinct parts that are combined on a macroscopic scale to produce a new material that has properties different from each of the individual components.
Typically, the objective is to have the new material exhibit the desirable properties from each of its constituents. In the case of composites for pipeline repair applications, the most common systems are fiber-matrix, wet-layup composites.
In a wet-layup system, a fabric material, typically composed of woven carbon or glass fibers, is saturated with a matrix material – often a thermoset resin (polyester, vinyl ester, water-activated polyurethane, etc.) – that cures or hardens via chemical reaction. The fibers making up the fabric material are what provide strength and stiffness to the composite repair in its final form. The matrix material is used to support, transport load between and protect the fibers.
Once the matrix material has fully cured, the cloth material that previously had little to no stiffness becomes a hard, rigid layer. Because the cloth remains formable during initial saturation (prior to cure), the composite system can be shaped into complex geometries that readily conform to molds or forms.
This characteristic of wet-layup systems is particularly attractive as an alternative to steel sleeves as they can be formed to closely fit pipeline geometries that may be ovalized or have some degree of curvature.
Since many of the defects being reinforced by composites change the profile of the pipe surface (e.g., external corrosion, dents, wrinkles), another hardenable nonmetallic material is used between the reinforcing fibers of the composite and the pipe profile.
This material is commonly referred to as the load transfer, or filler, material and serves as a rigid transition between the deformed pipe profile and the reinforcing fibers of the composite. The filler material has a high compressive strength and is critical in transferring load from the pipe to the reinforcement provided by the composite.
A pipeline composite repair typically will be comprised of many layers of the fiber-matrix material. For most applications, these layers are saturated and then wrapped circumferentially around the pipe. Each wrap, or ply, increases the thickness of the composite repair, which, in turn, increases the overall strength capacity of the repair once it has cured and, for the most part, can function as a single unit.
This is similar to increasing the wall thickness of the substrate pipe material. During installation, the number of wraps is increased until the design thickness is achieved. This design thickness is equal to the product of the thickness of each ply and the number of plies (wraps) installed.
Because the components of the composite repair are typically nonmetallic, they are influenced differently by variations in temperature, moisture or other contaminates. Unlike steel sleeve repairs, this can result in significant variability from system to system.
The structure of a composite repair also makes it heavily dependent upon the integrity of the load-transfer material – again, a fundamental concept that can be influenced by operating or installation variables. One of the easiest variables to overlook is surface preparation of the substrate; however, it is clear that the governing standards leave little room for interpretation of its importance.
Surface Preparation
It was identified that successful composite repairs act as a barrier between the external environment and the substrate being repaired, providing reinforcement to affected areas of the pipeline. Both functions are heavily reliant upon the bond between the composite and the substrate. If the composite is not sufficiently bonded, a pathway is created for the potentially corrosive environment to penetrate underneath the composite repair.
It is unlikely that the composite will fulfill its reinforcement task if a corrosion defect is allowed to grow, extend or interact with other anomalies. This could create an environment in which growth of the defect reduces the critical stress in the substrate to a point below what is achievable by the composite’s original design.
Additionally, this progression could disbond the composite from the substrate and allow deflection of the pipe before engaging the reinforcement of the composite. This allows for increased stress in the defect that could lead to exceeding the critical stress or a reduction in life if cyclic loads are present. Aside from implications as a protective barrier, poor adhesion could also prevent the transfer of axially oriented loads to the composite.
From a design consideration standpoint, an end-user should first look for guidance in the ASME (American Society of Mechanical Engineers) and ISO (International Organization for Standardization) standards referenced previously. Both are generally in agreement, though slight differences do exist. In the case of surface preparation, both place strong emphasis on its importance as shown in the select excerpts below:
PCC-2-2018
401-3.2.2 Surface Preparation
- The durability of a bonded assembly under applied load is determined by the quality of the surface preparation used.
- The specific method of surface preparation shall be an integral part of the repair system and its qualification.
- Any change in the surface preparation method requires requalification of the repair system.
ISO 24817 – 2015
Section 6 – Summary of Key Issues
- In terms of the performance of the repair system, the adhesion of the repair to the substrate is the key technical issue. The surface preparation procedure should be the same as that qualified by testing and assumed in the design.
Annex J.2 – Installation Requirements and Guidance
- Surface preparation is the single most important operation in the achievement of a successful repair. Surface preparation methods are not interchangeable. The procedure used for surface preparation is an integral part of a repair system, and an alternative preparation should not be used in lieu of that which has been qualified by the supplier.
Additional guidance on incorporating and documenting surface preparation is provided in each of the standards, including in the initial risk assessment of the repair. As an end-user, it is important to be reminded of the critical nature of this variable so that adequate oversight can be provided during installation.
Key installation variables do not stop at surface preparation. Operating conditions at the time of installation also have been shown to influence composite performance.
Operating Conditions
In addition to installation variables such as surface preparation, those related to pipeline operation can also play a significant factor in the success of a composite repair. For example, several studies have been performed to investigate the role of internal pressure during installation.
A study co-funded by the Bureau of Safety and Environmental Enforcement (BSEE) and the Pipeline Hazardous Materials Safety Administration (PHMSA) looked at the installation of composite repairs under various internal pressures when reinforcing plain dents and external wall loss defects.
It was observed that internal pressure appeared to have very little effect on the ultimate failure strength or life of the external wall loss repairs, but significantly reduced the life of the reinforced plain dents.
In the case of the plain dents, it was also noted that significant disbondment was observed between the load-transfer material, the pipe and the composite reinforcement. The observed differences are likely a result of the significant flexure that could be expected to coincide with a plain dent defect.
However, it is an important consideration for operators evaluating the feasibility of a composite repair. The ASME and ISO standards address internal pressure through a Plive variable found in certain design equations. (Plive is the internal pressure within the component during application of the repair.) Since the standards do not provide specific design equations for dents, cracks or other severe defect types, an end-user must incorporate engineering judgment, and often investigate empirically, for the influence internal pressure will have when repairing these types of features.
Post-Installation
Finally, a composite repair end-user also must understand expected future operating conditions of the affected pipeline. Internal pressure is the primary operational input into the majority of composite repair designs, but this also has aspects that are commonly overlooked.
For example, an end-user should consider incorporating future hydrostatic or spike test events into the repair design. It is not uncommon to utilize the pipeline’s MAOP/MOP (maximum allowable operating pressure/maximum operating pressure) as a design input.
Additionally, if a pipeline is operating at reduced pressures or reduced cycling severity at the time of installation, increases to either of these parameters could reduce the integrity of the repair over its design life.
Operating temperature is another consideration that can significantly change the expected performance of the composite. As discussed previously, pipeline composite repairs are typically nonmetallic and therefore more sensitive to temperature changes than a steel sleeve repair. For example, degradation of the filler material or reductions in composite strength are both possibilities for systems not designed to operate at elevated temperatures.
Both standards provide guidance for these considerations, and both incorporate these inputs into design equations. It is up to the end-user to determine the applicability of the selected inputs and, when necessary, look to engineering judgment when operating outside the guidance of the standards.
While no operator has a crystal ball to foresee all possible operating conditions, if experience shows that lines may be subject to changes in the items mentioned, it would be beneficial to consider these possibilities during the design of the composite as opposed to post-installation.
It is easy for fundamental principles to be overlooked in the rush from one issue to the next. Periodic reminders of their importance help to make consideration of the variables described front-end engineering questions, as opposed to post-installation headaches. In summary, an end-user should:
- Understand surface preparation used during material qualification and how it will be achieved during field installations. Measure and document surface preparation during installation in the field.
- When possible, reduce installation pressure at the time of installation to ensure the maximum level of reinforcement is achieved to reduce the likelihood of a disbondment.
- When reviewing design parameters for composite repairs, consider the possibility of changing future operations (e.g., increase in operating pressure) or performing hydrostatic or spike test.
Author: Colton Sheets is a professional engineer and senior associate at Stress Engineering Services, Inc. He holds a master of science degree from LeTourneau University and a bachelor of science degree from the University of Tulsa, both in mechanical engineering.
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